The proposed research concerns the biochemical mechanisms regulating growth and atrophy of skeletal muscle. These investigations will attempt to clarify the cellular events through which physiological factors, such as the level of muscular work, dietary intake and hormones induce muscle growth or cause muscle atrophy. This work represents a continuation of earlier studies of the regulation of amino acid transport, protein synthesis, protein degradation, and DNA synthesis in muscle. A major objective of future research will be to learn more about the regulation, selectivity, and physiological importance of intracellular protein breakdown in muscle and other cells. Relatively little is known at present about this important process, and we hope to elucidate the pathway for intracellular proteolysis and to identify the responsible degradative enzymes. Related studies will examine the control of amino acid metabolism in skeletal muscle. Muscle is the major site in the body for degradation of the branched chain amino acids and for the production of alanine and glutamine. We hope to learn more about the physiological significance of these processes and to clarify their regulation to one another and to the control of protein turnover in muscle.